Means for preventing or reducing the escape of high-frequency energy



May 24, 1949.

I ESCAPE OF HIGH-FREQUENCY ENERGY Filed Aug. 51, 1943 2 Sheets-Sheet 1ATTOPNEV J. COLLARD 2,470,805 MEANS FOR PREVENTING OR REDUCING THE May24, 1949. J. COLLARD MEANS FOR PREVENTING OR REDUCING THE ESCAPE OFHIGH-FREQUENCY ENERGY 2 Sheets-Sheet 2 Filed Aug. 31, i943 //v1/E/vT0,eloknfollard,

ATTORNEL Patented May 24, 1949 UNITED STATES PATENT OFFICE MEANS FORPREVENTING OR REDUCING THE ESCAPE OF HIGH-FREQUENCY ENERGY Great BritainApplication August 31, 1943, Serial No. 500,651 In Great BritainSeptember 12, 1941 Section 1, Public Law 690, August 8, 1946 Patentexpires September 12, 1961 6 Claims.

This invention relates to means for preventing or reducing the undesiredescape of high frequency energy.

It is common practice in connection with highfrequency apparatus toprovide conducting enclosures in screens for those parts which areliable undesirably to radiate high-frequency energy.

The escape of energy can be prevented where the conducting surface iscontinuous, but where even small gaps occur at the abutting edges of,for example, the lid of a screening box and the box itself, a leakagepath is provided permitting energy to escape at ultra-high-frequencies,particularly at centimeter wavelengths. Further, in some cases, it isnecessary to provide an aperture or apertures in said enclosure inscreen through which one or more conductors are passed. For example, theone or more conductors may comprise leads for supplying D. C. orlow-frequency A. C. to the high-frequency apparatus contained within ascreening box or said conductor or conductors may comprise a shaft orshafts to enable the apparatus within the screening box to bemechanically adjusted from the exterior of the box. In these cases also,at ultra-high-frequencies, undesirable escape of energy will occurthrough the said aperture or apertures.

The object of the present invention is to prevent or reduce the escapeof energy through a leakage path, due to a discontinuity or a conductingenclosure in screen, provided for example by a gap between the lid of ascreening boX and the box itself, or being due to the provision of anaperture or apertures.

According to one features of the invention there is provided apparatuscomprising a source of high frequency energy and a conducting enclosureor screen for said source wherein, in order to reduce substantially theundesired escape of said energy from a discontinuity in said enclosureor screen affording a leakage path, a path is provided branching fromsaid leakage path, said branch path being so dimensioned as to beresonant at said high frequency to provide a high impedance effectivelyin series with said leakage path.

Said leakage path is preferably so dimensioned as to be resonant at saidhigh frequency so as to present in effect a low impedance subsequent toand in series with said high impedance.

According to another feature of the invention there is provided aconducting enclosure or screen for high frequency energy wherein, inorder to reduce substantially the undesired escape of said energy from adiscontinuity in said enclosure or screen affording a leakage path, apath is provided branching from said leakage path, said branch path andpart or whole of said leakage path being both so dimensioned as to beresonant at a predetermined high frequency so as to present to theenergy which tends to escape a high impedance in series with asubsequent low impedance.

According to another feature of the invention tin is provided apparatuscomprising a source of high frequency energy and a conducting enclosureor screen for said source, said enclosure having a leakage path throughwhich an undesired escape of high frequency energy can occur, whereinsaid leakage path is made of a length so as to be resonant at said highfrequency so that it presents a low impedance to the energy that tendsto escape, whereby the escape of said energy is reduced.

branch path may take a variety of forms as will be hereinafter morefully referred to, but preferably said branch path is effectivelysubstantially a quarter of a wavelength long at said predeterminedfrequency and is short-circuited at one end.

If desired, said leakage path prior to said branch path can be maderesonant to the energy that tends to escape so as to present at thecommencement of said leakage path a low impedance whereby the energywhich is permitted to flow into said leakage path is smaller than wouldotherwise be the case.

In order that the invention may be clearly understood and readilycarried into effect, various embodiments of the invention will now bedescribed with reference to the accompanying drawings in which:

Figure 1 shows a cross-sectional view of a part of a screen for housinghigh-frequency apparatus and embodying one construction according to theinvention for reducing or eliminating escape of energy through anaperture in a screen through which a conductor passes,

Figure 2 is a view similar to Figure 1 illustrating a modification,

Figure 3 illustrates a form of the invention similar to that shown inFigure 1 as applied to a screen in which a pair of concentric shaftsproject through an aperture in said screen,

Figure 4 shows diagrammatically in cross-section a simple form ofscreening box embodying the invention for the purpose of reducing oreliminating the escape of energy between the lid of the screening boxand the box itself, and

Figures and 6 show modifications of the construction shown in Figure l,

Figure '7 is a modification of the construction of screening box shownin Figure 4 with a view to providing a more compact construction,

Figure 8 is a cross-sectional view of a portion of a screening boxembodying a further form of the invention, and

Figure 9 is a view similar to Figure 8 illustrating a modification ofthe arrangement shown in Figure 8. 7

Referring first to Figure 1 of the drawings, the reference numeral 3indicates a portion of a conducting screen for surroundinghigh-frequency apparatus, not shown, the screen having an aperture 4through which extends a lead 5 serving to supply D. C. or low-frequencyA. C. to parts of the apparatus enclosed within the screen 3, theapparatus being assumed to lie on the left-hand side of the screen asviewed in the drawing. With such an arrangement it will be found thathighfrequency energy will escape through the leakage path afforded bythe discontinuity, i. e., the aperture 4 in the screen. In order toreduce or eliminate this escape of energy a conducting sheath 6 isprovided surrounding the lead 5 connected at one end to the screen 3 inthe vicinity of the aperture 4. Surrounding the lead 5 in the exampleshown, are two tubular conductors I and 8 each closed at one end andconnected at said end to the lead 5 so providing branch paths forhighfrequency energy tending to escape through the aperture 4. The spacebetween the sheath 6 and the tubular conductors l and 8 and the spacebetween the latter and the lead 5 may be filled, if desired, withsuitable insulating material, although this is not essential sinceair-spacing of these elements is sufficient. The effective length ofeach tubular conductor l and 8 is such as to give quarter wave resonanceat the frequency of the high-frequency energy that tends to escape, thephysical length of those conductors depending, of course, on thedielectric medium between the conductors and the lead 5. The length ofthe sheath 6 is sufficient to enclose both the tubular conductors l andt, and although in the example shown two tubular conductors areemployed, it will be appreciated that, if desired, only a single tubularconductor, or more than two tubular conductors, may be used, the lengthof the sheath 6 being suitably chosen accordingly.

It will be observed from Figure 1 that there is no direct connectionbetween the lead 5 and the screen ('3 so that the lead 5 is insulatedfrom the screen so far as D. C. or low-frequency A. C. is concerned. Anyhigh-frequency energy, however, which tends to flow towards the aperture4 through the sheath 6 is confronted by the branch path formed by theconductor '5. Since the length of the conductor l is such as to resonateat the high frequency which it is desired to suppress and isshort-circuited Where it is connected to the lead 5, the impedancepresented by the branch path to the energy which tends to escape is veryhigh. The path between the sheath 6 and the conductor 1, which is partof the leakage path, is so dimensioned as to resonate at said highfrequency as an open-circuited quarter wave line and hence itsimpedance, as viewed from the lefthand end of the lead 5 is very low andis in series with and subsequent to the high impedance referred to. Theenergy which tends to escape is thus confronted by a high impedance inseries with a low impedance and hence substantial attenuation occurs.The energy which ultimately escapes past said high and low impedances isagain confronted by two similar impedances, one of these, namely thecircuit presented by the lead 5 and the tubular conductor 8,constituting a second branch path which presents a high impedance. Theother, namely the circuit constituted by the tubular conductor 8 and thesheath 6 is a further portion of the leakage path and is of lowimpedance so that further attenuation of high-frequency energy occurs.If it is found that two tubular conductors i and 8 with theirsurrounding sheath 6 are insufiicient to produce a sufficientattenuation, further tubular conductors may be employed so as to obtainany desired attenuation. Thus since the escape of energy from theaperture ii is substantially prevented, the fiow of high frequencycurrent along the lead 5 is also substantially prevented It is, ofcourse, immaterial which end of each tubular conductor is connected tothe lead 5 and, furthermore, it is immaterial whether the sheath 6 andthe one or more tubular conductors project within the screen 3 orproject on the exterior of the screen 3. Furthermore, it is notessential to short-circuit the tubular conductors l and 8 t0 the lead 5since, as shown in Figure 2, these conductors can be short-circuited tothe sheath 6. The operation of the construction shown in Figure 2 issimilar to that described with reference to Figure 1, although in thiscase the high impedance circuits are presented by the tubular conductors short-circuited to the sheath 6 and the low impedance circuitsare presented by the lead 5 and the tubular conductors l and 8. It willof course be understood that in Figure 2 the low impedance circuits willstill be present even if the lead 5 were omitted.

In Figure 3 an arrangement similar to that shown in Figure 1 describedabove is illustrated as applied to shafts projecting through a screen 3,the shafts being employed for mechanically adjusting apparatus containedwithin the screen. In this Figure the shafts project through theaperture t in the screen 3, one shaft being indicated by the referencenumeral 9 arranged concentrically with respect to the aperture, whilstthe other shaft ll) is of tubular construction, concentric andsurrounding the shaft 9. Such shafts may, for example, be employed forthe coarse and fine adjustment of a hollow resonator. In order to reduceescape of high-frequency energy between the shafts 9 and ll] and theaperture l, tubular conductors H, I2 and I3 are attached to the shaft 9and tubular conductors I4, l5 and l 6 are attached to the shaft l8,these conductors being similar to the conductors l and 8 described withreference to Figure 1. The function of the construction shown in Figure3 of the accompanying drawings will, it is thought, be readilyappreciated sinc it is analagous to the operation of Figure 1.

Figure 4 of the drawings illustrates the invention as applied to acircular metal screening box. Such forms of screening box require thatthe lid of the box be removably mounted in position to permit of accessto the apparatus within the box. Providing the lid to the box couldbemade to fit perfectly to the base of the box no power could escape.However, such a fit is not practicable and, consequently, owing to thediscontinuity in the screening between the lid and the base of the boxdue to the small gaps which will inevitably result owing to theimperfect fit, a leakage path will resultand high-frequency energy willescape. In order to reduce or eliminate escape in this fashionwhilststill permitting the lid of the box to be readily removable thelid L is arranged to extend between the coaxial bands r1, r2, solderedto the base B of the box. The surfaces of the lids and bands r1, r2,overlap in such a manner that the distances at to b, b to c and b to clindicated in the drawing are made equal to a quarter of a wavelength atthe operating frequency of the high-frequency apparatus contained withinthe screening box. The branch path in this figure is the path b to cwhich is effectively a quarter wave-length path, short-circuited at thelower end and thus constitutes a high impedance, as seen from the pointb so that in combination with the path from b to d which constitutes alow impedance as seen from b it serves to reduce high frequency energyescaping along the path between D and d. Due to the surface a to b whichoverlaps the lower edge of the lid L, the leakage path, in effect isextended prior to said branch path and is so dimensioned, i. e., thisportion of the leakage path is a quarter of a wavelength long as topresent at the commencement of the leakage path i. e. at a a lowimpedance whereby the energy which is permitted to flow into saidleakage path is less than would otherwise be the case.

The principle illustrated in Figure 4 may be extended in the mannershown in Figure 5 and 6 in order more effectively to reduce the escapeof energy. It will be seen from Figure 5 that additional quarterwavelength branch paths are provided and the leakage path prior andsubsequent to these additional branch paths are extended as with thearrangement of Figure 4. Figure 6 is a modification which is less bulkyfrom the mechanical point of view. A plurality of quarter wavelengthbranch paths are arranged in series along the wall of the lid andco-operate with a common surrounding band, as shown. In this figure,however, the leakage paths of low impedance prior to the branch pathsare not employed and the operation is similar to that described withreference to Figure 1. i

The overall height of a screening box and the co-operating bands may beconveniently reduced if desired by using a solid dielectric materialhaving a large dielectric constant (K) as the medium between theoverlapping surfaces of the box and bands, or the edge of the box mayrest upon such a dielectric material. The requisite quarter wavelengthpath may thus be reduced in the ratio In applying such a proposal to thescreening box shown in Figure 4, the space between the points a and c isfilled with a suitable dielectric material and the distances at to b, bto c and b to d lessened accordingly, although still effectively equalto quarter wavelengths. Alternatively, the space between b and 0 may befilled with a solid dielectric material and the edge of the lid L maythen rest on the dielectric. The distances a. to b and b to d in thiscase will still be equal to a quarter of a wavelength with air as thedielectric, but the distance b to 0 will be reduced depending upon thedielectric constant of the material selected, although still presentinga quarter wavelength path.

Figure 7 illustrates a further form of screening box in which the lid Lcan rest directly on the base B of the box and projects between twobands r1, 1-2, the band r2, however, being of the form illustrated, thatis to say, it projects upwardly from the base B and is provided with anextension 13 which extends between the band T2 and the lid L. Theconstruction shown in this figure provides a single branch path with theleakage path extended prior and subsequent to the branch path as inFigure 4.

Figure 8 of the accompanying drawings illustrates a further embodimentof the invention as applied to a screening box of circular or othersuitable form, the box comprising two portions l8 and IS] the peripheryof the sections l8 and [9 being enlarged as shown in cross-section inthe figure. The portions 18 and 19 are shown spaced apart at theirenlarged peripheries, but in practice these adjacent surfaces may abut.In order to reduce or prevent the escape of high frequency energythrough the leakage path formed by the abutting surfaces, an annularchannel 20 is formed in the portion [8, this channel extending aroundthe periphery of the portion [8. The channel 28 forms a branch path andthe length of the said path is arranged to be a quarter of a wavelengthat the operating frequency of the high frequency apparatus containedwithin the screening box. The branch path thus presents effectively ahigh impedance in series with the leakage path and serves to effectsubstantial attenuation of high frequency energy. The length of theleakage path indicated at 21 prior to the branch path 20 is alsodimensioned to be a quarter of a wavelength long so that the impedanceat the commencement of the branch path is low and hence the amount ofenergy which can pass through the leakage path is smaller than wouldotherwise be the case. The portion of the leakage path indicated at 22subsequent to the branch path 2% is also dimensioned to be a quarter ofa wavelength long so as to present as low an impedance as possible ineffective series relation with said high impedance so that leakagethrough said leakage path is lower than would otherwise be the case.

Figure 9 is a modification of the arrangement shown in Figure 3. In thisfigure the arrangement of branch and leakage paths 20, 2| and 22 issimilar to that described with reference to Figure 3 except that theirdisposition is changed relatively to the screening box to enable theperiphery of the portion l8 to enclose the periphery of the portion [9.

Although in the various embodiments described above reference has beenmade to paths which are a quarter of a wavelength long, it will beunderstood that the invention is not limited to paths of this lengthsince it will be obvious that the dimensions of the paths may besubstantially equal in length to an odd number of such quarterwavelengths, which length is effectively equal to a quarter of awavelength so as to be resonant at the frequency of the energy whichtends to escape.

If desired, such branch path may be made effectively equal to ahalf-wavelength of the highfrequency energy so as to be resonant at thefrequency of th energy which tends to escape, said branch path howeverbeing open-circuited at its end remote from the leakage path, so as toprovide said high impedance. In such a case, in order to prevent energyfrom escaping at the open-circuited end of said branch path, the latterend may open into a cavity of suitable dimensions. Such an arrangementcould be applied to the embodiment shown in Figure 3 of the accompanyingdrawings.

What I claim is:

1. Means for preventing the escape of high frequency energy through adiscontinuity in a screen for said energy, there being a conductingmember passing through said discontinuity, said discontinuity and saidconductive member providing a leakage path through said screen, saidmeans including a conducting sheath member surrounding said conductingmember and connected to said screen, a supplementary conductor withinand lying parallel to said sheath and connected to one of saidconducting members, said supplementary conductor having an electricallength of the order of one quarter of the wavelength of said highfrequency energy.

2. Means for preventing the escape of high frequency energy through adiscontinuity in a screen for said energy, said discontinuity providinga leakage path through said screen, said means including a number ofbranch paths in series with said leakage path, said branch paths eachhaving an electrical length substantially equal to one quarter of thewavelength of said high frequency energy, and being short-circuited atthe end remote from the point of connection in series with said leakagepath at least one of said branch paths being connected to said leakagepath at a distance equal to one-quarter of the wavelength of said highfrequency energy from the interior of said screen.

3. A conductive enclosure for high frequency energy, said enclosurehaving an apertur therein forming a leakage path for the escape of saidenergy, said leakage path having one portion communicating with theinterior of said enclosure and having an electrical length substantiallyequal to an odd integral number including unity of quarter wavelengthsat the frequency of said energy and another portion communicating withthe exterior of said enclosure and having an electrical lengthsubstantially equal to an odd integral number including unity of quarterwavelengths at the frequency of said energy, said portions beingresonant at the frequency of said energy tending to escape to provide alow impedance thereto, and at least one branch path connected in serieswith said leakage path at a point intermediate said portions, saidbranch path presenting a high impedance to said energy tending to escapeover said leakage path.

4. A conductive enclosure for high frequency energy, said enclosurehaving an aperture therein forming a leakage path for the escape of saidenergy, at least one branch path connected in series with said leakagepath at a point intermediate the ends thereof, said branch pathpresenting a high impedance to said energy tending to escape over saidleakage path, and said leakage path having dimensions at Which eachportion thereof as measured from said point to the respective endsthereof has an electrical length substantially equal to an odd integralnumber including unity of quarter wavelengths at the frequency of saidenergy and is resonant at the frequency of said energy tending to escapeto provide a low impedance thereto.

5. An enclosure for high frequency energy including a conductiveenclosing wall for maintaining said energy therein, there being anaperture in said enclosing wall forming a leakage path for the escape ofsaid energy, first and second conductive walls arranged in concentricrelationship and connected to said conductive enclosing wall, saidconducting walls being spaced apart and having dimensions at which abranch path having a high impedance to the energy tending to escapealong said leakage path is connected in series with said leakage path ata point intermediate its length and substantially an odd multipleincluding unity of quarter wavelengths at the frequency of said energyfrom each end of said leakage path and said leakage path issubstantially a quarter wavelength long at the frequency of said energy.

6. An enclosure for high frequency energy including a first conductiveenclosing Wall and a second conductive enclosing wall in end-to-endabutting relationship, the aperture between said walls forming a leakagepath for the escape of said energy, the abutting portion of said wallshaving a width substantially equal to a half wavelength at the frequencyof said energy, and at least one of said walls having a slot thereinmidway of the width of said wall, said slot having a depth ofsubstantially a quarter wavelength at the frequency of said energy.

JOHN COLLARD.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,226,479 Pupp Dec. 24, 19402,247,731 Olson July 1, 1941 2,247,746 Burt July 1, 1941 2,296,678Linder Sept. 22, 1942 2,322,971 Roosenstein June 29, 1943 2,332,952Tischer et a1 Oct. 26, 1948 2,342,254 Dallenbach Feb. 22, 1944 2,351,895Allerding June 20, 1944 2,411,299 Sloan Nov. 19, 1946 OTHER REFERENCESPractical Analysis of Ultra High Frequency by Meagher and Markley, pub.by R. C. A. Service Co., Inc., Camden, New Jersey, Aug. 1943. See page16.

, Inns

